Wenn der Rauchmelder wieder piept, rufe ich den Hausmeister an.

If the smoke detector beeps again, I’ll call the caretaker.

Breakdown of Wenn der Rauchmelder wieder piept, rufe ich den Hausmeister an.

• ich: I
• wieder: again
• anrufen: to call
• den: the; (masculine, accusative)
• der Hausmeister: the caretaker
• wenn: if/when
• der Rauchmelder: the smoke detector
• piepen: to beep

Questions & Answers about Wenn der Rauchmelder wieder piept, rufe ich den Hausmeister an.

Why does the verb come first in Wenn der Rauchmelder wieder piept?

Because wenn starts a subordinate clause. In German subordinate clauses, the conjugated verb goes to the end of the clause.
So it’s: wenn + (subject/object/etc.) + verb → Wenn der Rauchmelder wieder piept.

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Why is there a comma after the wenn-clause?

German normally requires a comma between a subordinate clause and the main clause. Here, the subordinate clause comes first, so you must separate it with a comma:
Wenn … piept, rufe ich … an.

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Why is it rufe ich and not ich rufe?

Because when a subordinate clause comes first, it takes the first position in the sentence. The main clause then follows V2 (verb-second) word order, meaning the conjugated verb comes next, and the subject comes after it:
Wenn … piept, rufe ich …
(Here the verb is second overall, and ich is third.)

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What’s the function of wieder here, and where can it go?

wieder means again (or once more). In this sentence it modifies piept (beeps again).
Its placement is flexible, but the given position is very natural: Wenn der Rauchmelder wieder piept.
You could also say Wenn der Rauchmelder piept, wieder rufe ich… but that would sound unnatural and change emphasis.

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Why is it der Rauchmelder (masculine)? What does the word mean exactly?

Rauchmelder is a compound: Rauch (smoke) + Melder (detector/alert device). It means smoke alarm / smoke detector.
The head word is Melder, which is masculine (der Melder), so the compound is masculine too: der Rauchmelder.

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Is piepen the usual verb for a smoke alarm? Could I also say klingeln or läuten?

Yes, piepen is very common for devices making short high-pitched beeps (alarms, electronics).

• piepen = to beep (typical for smoke alarms)
• klingeln = to ring (often for doorbells, phones)
• läuten = to ring (especially bells, church bells; also phones in some contexts)
  For a smoke alarm, piepen (or sometimes Alarm schlagen, losgehen) is the most idiomatic.

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Why is it den Hausmeister (accusative), and what does anrufen require?

Because anrufen (to call (someone)) takes a direct object in the accusative:

• ich rufe den Hausmeister an = I call the caretaker.
  So der Hausmeister becomes den Hausmeister in the accusative.

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What is Hausmeister, and is it the same as a landlord?

A Hausmeister is typically a building caretaker / superintendent / janitor—someone who handles repairs, maintenance, and building issues.
It’s not necessarily the landlord (der Vermieter), though in small buildings one person can sometimes fill multiple roles.

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Why is an separated and placed at the end: rufe … an?

anrufen is a separable verb (an- + rufen). In a main clause, the prefix goes to the end:

• Ich rufe dich an.
  But in a subordinate clause, it stays attached at the end:
• …, weil ich dich anrufe.

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Could this sentence also mean “Whenever the smoke alarm beeps…”?

Yes. Wenn can mean if or when, and in many contexts it also works like whenever (repeated situations).
Here, with wieder (again), it strongly suggests a repeated event: If/when it beeps again, I’ll call the caretaker.

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Why is the main clause in present tense rufe ich if it refers to the future?

German often uses the present tense to talk about the future when the future meaning is clear from context (here: a conditional future situation).
So rufe ich can mean I will call. You could also use werde … anrufen, but it’s less natural here and can sound more formal or emphatic.

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Can the order be reversed: Ich rufe den Hausmeister an, wenn …?

Yes, and then the main clause uses normal word order:
Ich rufe den Hausmeister an, wenn der Rauchmelder wieder piept.
The meaning is basically the same; the difference is mainly what comes first (topic/emphasis).